Densitometers are generally known in the art and are used to measure a density of a fluid. The fluid may comprise a liquid, a gas, a liquid with suspended particulates and/or entrained gas, or a combination thereof. While there are various types of densitometers that operate according to different principles, one type of densitometer that has received great commercial success is a vibrating densitometer. Vibrating densitometers can comprise a vibrating member, such as a cylinder, a conduit, a pipe, a tube, etc. that is exposed to a fluid under test. One example of a vibrating densitometer comprises a conduit cantilever mounted with an inlet end coupled to an existing pipeline or other structure and the outlet end free to vibrate. Alternatively, both the inlet and outlet may be fixed with the portion of the conduit between the inlet and outlet vibrating. The conduit can be vibrated at resonance and the resonant frequency can be measured. As is generally known in the art, the density of the fluid under test can be determined by measuring the reduced resonant frequency of the conduit. According to well-known principles, the resonant frequency of the conduit will vary inversely with the density of the fluid that contacts the conduit. Therefore, while some vibrating densitometers are capable of measuring a density of a liquid, viscous damping caused by the liquid on the outside of the cylinder can reduce the measurement capabilities of vibrating densitometers. Liquid vibrating densitometers thus use vibrating pipes or tubes that have the fluid only on the inside, while gas vibrating densitometers typically are immersed in the fluid, having gas both on the inside and the outside of the cylinder. Therefore, typically, vibrating densitometers are used to measure a density of a gas.
FIG. 1 shows a prior art immersed densitometer 10. The prior art densitometer 10 may be configured to measure a density of a fluid, such as a liquid or a gas, for example. The densitometer 10 includes a housing 11 with a vibrating member 12 located at least partially within the housing 11. A portion of the housing 11 is cut away to show the vibrating member 12. The densitometer 10 may be placed in-line in an existing pipeline, for example. Alternatively, the housing 11 may comprise closed ends with apertures to receive a fluid sample, for example. Therefore, while flanges are not shown, in many instances, the housing 11 or the vibrating member 12 may include flanges or other members for operatively coupling the densitometer 10 to a pipeline or similar fluid delivering device in a fluid-tight manner. According to the example shown, the vibrating member 12 is cantilever mounted to the housing 11. The vibrating member 12 is shown coupled to the housing 11 at an inlet end 13 with the outlet end 14 free to vibrate.
According to the example shown, the vibrating member 12 also includes a plurality of fluid apertures 15 near the inlet end 13. The fluid apertures 15 can be provided to allow some of the fluid entering the densitometer 10 to flow between the housing 11 and the vibrating member 12. Therefore, the fluid contacts the inside as well as the outside surfaces of the vibrating member 12. This is particularly helpful when the fluid under test comprises a gas because a greater surface area is exposed to the gas. In other examples, apertures may be provided in the housing 11 to expose the fluid under test to the outer surface of the vibrating member 12 and therefore, the apertures 15 are not required in the vibrating member 12.
Further shown in FIG. 1 is a driver 16 and a vibrating sensor 17 positioned within a cylinder 50. The driver 16 and vibrating sensor 17 are shown as comprising coils, which are well known in the art. If an electric current is provided to the coil, a magnetic field is induced in the vibrating member 12 causing the vibrating member 12 to vibrate. Conversely, the vibration of the vibrating member 12 induces a voltage in the vibrating sensor 17. The driver 16 receives a drive signal from a meter electronics 18 in order to vibrate the vibrating member 12 at one of its resonant frequencies in one of a plurality of vibration modes, including for example simple bending, torsional, radial, or coupled type. The vibrating sensor 17 detects the vibration of the vibrating member 12, including the frequency at which the vibrating member 12 is vibrating and sends the vibration information to the meter electronics 18 for processing. As the vibrating member 12 vibrates, the fluid contacting the vibrating member's wall vibrates along with the vibrating member 12. The added mass of the fluid contacting the vibrating member 12 lowers the resonant frequency. The new, lower, resonant frequency of the vibrating member 12 is used to determine the density of the fluid as is generally known in the art according to a previously determined correlation, for example.
As is generally known, to obtain accurate density measurements, the resonant frequency used to measure the density of the fluid must be very stable. This is particularly true when the fluid comprises a gas as the resonant frequency changes by a smaller amount compared to liquid. One prior art approach to achieve the desired stability is to vibrate the vibrating member 12 using a radial vibration mode. In contrast to a bending vibration mode, for example, where the longitudinal axis of the vibrating member translates and/or rotates away from its rest position, in a radial vibration mode, the longitudinal axis of the vibrating member remains essentially stationary while at least a part of the vibrating member's wall translates and/or rotates away from its rest position. Radial vibration modes are preferred in straight conduit densitometers, such as the prior art densitometer 10 shown in FIG. 1 because radial vibration modes are self-balancing and thus, the mounting characteristics of the vibrating member are not as critical compared to some other vibration modes. One example radial vibration mode is a three-lobed radial vibration mode. An example of the change in shape of the vibrating member's wall during a three-lobed radial vibration mode is shown in FIG. 3.
If the vibrating member 12 is perfectly round and has a perfectly uniform wall thickness, there is only one three-lobed radial vibration mode. However, due to design tolerances, this is usually impractical. Consequently, when a manufacturer attempts to make a perfectly round vibrating member 12 with a perfectly uniform wall thickness, small imperfections result in two three-lobed radial vibrations that vibrate at two different resonant frequencies, which are very close to one another. The three-lobed radial vibrational mode with the lower resonant frequency will vibrate with the peaks and valleys as shown in FIG. 3 aligned with the thinner walled portions while the higher frequency will vibrate with the peaks and valleys at the thicker wall portions. The frequency separation between the two modes is typically very small and may be less than a hertz. With two resonant frequencies so close together, a density determination is impractical because an operator will often not be able to distinguish the vibrational frequencies to determine what mode is being driven into vibration and thus, the correct density.
In some prior art densitometers, this problem is addressed by tuning the radial mode so that it has at least a minimum frequency separation between the two three-lobed radial vibrational modes as well as from the other vibration modes, such as the two lobed modes or the four lobed modes. While the tuning can be accomplished according to a variety of techniques, one prior art approach tuning method is by grinding the vibrating member's wall in axially aligned strips so the vibrating member has different thicknesses in different circumferential regions. This is shown in FIG. 1, but in more detail in FIG. 2.
FIG. 2 shows the vibrating member 12 taken along line 2-2 of FIG. 1. FIG. 2 is shown with reference angles shown as well. The reference angles are taken where the driver 16 and the vibrating sensor 17 are positioned at 0°. However, the angles are merely shown as an example and other reference coordinate angles may be used.
As shown, the vibrating member 12 comprises varying wall thicknesses around the circumference of the conduit. For example, the vibrating member 12 may originally comprise a thickness of approximately 0.005 inches (0.125 mm). The driver 16 and the vibrating sensor 17 are centered on one of these thick walled regions. Starting at approximately 15° and spacing uniformly around the circumference of the vibrating member 12 at approximately 30° intervals, six regions of the wall of the vibrating member 12 are ground down to approximately 0.004 inches (0.100 mm). Typically, the thickness of the wall is reduced by using a mandrel that has movable segments moved into position by hydraulic pressure. When the mandrel is pressurized, the movable segments move out the required amount to contact the vibrating member 12 and the thinner regions are ground. By grinding the vibrating member wall thickness in various circumferential regions, the resonant frequencies of the two three-lobed radial vibration modes are separated from one another. With the spacing between the thin regions being approximately 30°, the higher frequency three-lobed radial mode will be offset from the lower frequency three-lobed radial mode by approximately 15°. In one example, the lower frequency three-lobed vibrational mode will vibrate with the peaks and valleys centered on the thin and thick portions while the higher frequency three-lobed vibrational radial mode will have the peaks and vales half-way between the thin and thick regions.
The above-mentioned process has several problems. The hydraulic mandrel is at the limit of its dimensional capability. In other words, the grinding is extremely precise and is often close to or even beyond the design capabilities of the hydraulically operated mandrel. Further, non-destructive measurement of the vibrating member wall thickness is extremely difficult. Consequently, the acceptable product yield is low, resulting in an increased cost associated with the tuning method. In addition, in order to drive the lower frequency three-lobed radial vibrational mode, the driver 16 needs to be positioned opposite the thick walled portion. However, the difference between the thick and thin walled portions is typically extremely small at 0.001 inches (0.025 mm). Therefore, proper placement is very difficult. Further, as shown in FIG. 2, it is extremely difficult to match the grinding of each of the thin walled portions to one another.
Therefore, there exists a need for a method and apparatus for improving vibrating densitometers. Specifically, there exists a need for a vibrating densitometer with increased vibration mode separation while maintaining a higher product yield. The present invention solves this and other problems and an advance in the art is achieved.